1. Field of the Invention
The present invention relates to an apparatus for optically recording and reproducing information, and more particularly to an optical information recording and reproducing apparatus which is suitable to a magnetic field modulation system.
2. Related Background Art
An optical memory such as an optical disk or an optical card has been recently attracting notice as a large capacity memory having a low cost per bit. Among others, a magneto-optical disk, which is a magneto-optical recording medium that permits erasing of information and an application thereof to an external recording unit of a computer and a memory for an image file, has been extensively studied.
The magneto-optical disk recording system includes a light modulation system and a magnetic field modulation system. In the light modulation system, an intensity-modulated laser beam is irradiated while a constant biasing magnetic field is applied to the magneto-optical disk. In the magnetic field modulation system, a modulated biasing magnetic field is applied while a constant intensity laser beam is irradiated. Of those two systems, in order to overwrite information, that is, record new information while erasing previously recorded information, the magnetic field modulation system is advantageous because the medium structure of the magneto-optical disk may be simple. A common information recording method is a mark interval recording method in which an interval (or position) of a mark called a domain on the magneto-optical disk is modulated to record information. On the other hand, a mark length recording method which can increase a recording density by approximately 1.5 times by modulating a length of the mark to record information has also been studied. In high density recording by the mark length recording method, the magnetic field modulation system is also advantageous because it is hardly restricted by the size of a light spot or light intensity distribution.
FIG. 1 shows a configuration of a magneto-optical disk and peripheral units in the magnetic field modulation system. In FIG. 1, numeral 7 denotes a semiconductor laser used as a light source. A laser beam thereof is collimated .by a collimator lens 6 and converted to a circular light beam by a beam shaping prism 5. It passes through a beam splitter 4, is focused by an objective lens 3 and is directed to a recording layer of a magneto-optical disk 2 as a fine light spot. A magnetic head 1 is arranged above the magneto-optical disk 2 to face the objective lens 3.
On the other hand, the light reflected by the magneto-optical disk 2 passes through the objective lens 3 and the beam splitter 4 and is directed to a beam splitter 9, which splits the reflected light into two light beams, one of which is directed to a reproducing optical system through a one-half wavelength plate 10 and the other is directed to a controlling optical system through a condenser lens 15. In the reproducing optical system, the light transmitted through the one-half wavelength plate 10 is directed to a beam splitter 12 through a condenser lens 11 and it is further split into two light beams. The split light beams are detected by photo-detectors 13 and 14, respectively, and an information signal is derived from the detection outputs. In the controlling optical system, the light transmitted through the condenser lens 15 is split into two light beams by a beam splitter 16. One of the split beams is detected by a photo-detector 19 and the other is detected by a photo-detector 18 through a knife edge 17. A servo signal is derived from the detection signals of the photo-detectors 19 and 18.
When information is to be recorded, a laser beam of a constant intensity is irradiated by the semiconductor laser 7 to apply a thermal bias to the magneto-optical disk 2. As a result, temperature of the recording layer rises beyond a Curie point. Under this state, a biasing magnetic field is applied from the magnetic head 1 to the high temperature region. The biasing magnetic field is modulated by a recording signal and the orientation of magnetization of the recording layer is aligned to the direction of the biasing magnetic field. When the temperature of the recording layer goes down below the Curie point, the orientation of magnetization is held and a domain is formed. Since the light spot usually has a Gaussian distribution of light intensity, a temperature distribution of the heated medium reflects the light intensity distribution and it is gentle. Thus, an isothermal line of the Curie point is not rectangular. As a result, the recorded domain has an arrow shape as shown in FIG. 2A and 2B. FIG. 2A shows domains recorded by the mark interval recording method. The mark shapes are equal and the mark intervals are modulated. FIG. 2B shows domains recorded by the mark length recording method. The length of the marks are modulated and the edge intervals represent the information.
In the prior art magnetic field modulation system, however, when the arrow-shaped domain which corresponds to an information bit is to be reproduced, the arrow-shaped domain is scanned by the circular light spot having the Gaussian distribution in the light intensity distribution. As a result, correlations between the light spot and the domain are different after the arrow and the before the arrow, crosstalk from front and rear adjacent domains are asymmetric, and the sharp portion of the arrow-shaped domain, that is, a high spatial frequency portion is a stable in shape. Accordingly, because of those problems and a poor MTF of the optical system, the prior art apparatus cause an increase in signal jitter, an increase in error rate and a decrease in reliability. Particularly, in the mark length recording method shown in FIG. 2B, the increase in error rate caused by jitter is remarkable because the edge position of the domain represents the information.